This invention relates to a system for monitoring an air pressure of a vehicle tire with which the operator in a vehicle can monitor the air pressures of the tires even during traveling of the vehicle.
If a motor vehicle runs continuously for a long time with the tires considerably low in air pressure, the temperature of the tires may increased.
In order to overcome this difficulty, it is necessary to monitor the air pressures of the tires during traveling of the vehicle. However, since the tires are rotated at high speed during traveling of the vehicle, special means must be provided. For this purpose, the following air pressure monitoring means has been proposed according to the prior art: A pressure sensor which operates when the air pressure of a tire becomes lower than a predetermined value, and a transmitter for transmitting a radio wave signal in response to the operation of the pressure sensor are provided on the tire wheel side; while a receiver for receiving the radio wave signal, and an alarm unit for notifying the operator in response to the reception of the signal that the tire air pressure has been decreased.
The above-mentioned conventional air pressure monitoring means is disadvantageous in that it is necessary to employ the transmitter and the receiver which are expensive and relatively undurable against vibration. Furthermore, the operation of the air pressure monitoring means may be affected by spark noises or by electrical noises from radio equipment on other vehicles.
If the same air pressure monitoring means is installed on another vehicle, than the radio wave signals from the air pressure monitoring means of the two vehicles may interfere with each other.
On the other hand, of a plurality of tires of a vehicle, one whose air pressure has decreased can be located by differently coding the radio wave signals transmitted by the different transmitters. However, in the case where the tires are relocated for so-called "tire rotation", the coded signals must be changed. This coded signal changing operation is rather troublesome, which lowers the practicability of the method.
Furthermore, in the conventional air pressure monitoring means, the battery provided on the load wheel side is limited in capacity; that is, its service life is relatively short, and therefore the battery must be frequently replaced.